The invention relates to maskless lithography using a multiplexed array of Fresnel zone plates.
Lithography is conventionally performed by a variety of systems and methods. Optical projection lithography employs a reticle (also called a mask) which is then imaged onto a substrate using either refractive or reflective optics, or a combination of the two. The reticle or mask contains the pattern to be created on the substrate, or a representation thereof. Often, but not always, the optics produces a reduction of the reticle image by a factor between 4 and 10. In other cases there is no reduction of magnification, often referred to as 1-to-1 imaging.
X-ray lithography employs a mask held in close proximity (e.g., a gap of zero to 50 micrometers) to the substrate. By passing x-ray radiation through the mask, the pattern on the mask is replicated in a radiation-sensitive film on the substrate. This film is commonly called a "resist".
Electron-beam lithography is often carried out by scanning a well focused electron beam over a substrate coated with a resist. By turning the beam on and off at appropriate times, in response to commands from a control computer, any general 2-dimensional pattern can be created. This form of electron-beam lithography is referred to as a "maskless lithography", since no mask is employed. The pattern is created sequentially, dependent upon commands from a control computer.
Another form of maskless lithography, commercially available from ETEC Corporation of Hayward, Calif., employs an array of light beams which scan across a substrate and are shut on and off in response to commands from a control computer. This system is referred to as an optical pattern generator. The resolution of this system is limited by the numerical aperture of the lenses used and the wavelength of the ultraviolet radiation, in accordance with a well-known relationship: EQU p=.lambda./NA
where p is the minimum resolvable period, .lambda. is the wavelength of the radiation, and NA is the numerical aperture of the lens.
Lithography methods that create a pattern via commands from a control computer have a significant advantage over those that require a mask.
X-ray lithography is considered an attractive approach to manufacturing semiconductor products with minimum size features of 100 nm and below, because of its capabilities for high resolution. For example, linewidths as narrow as 18 nm have been replicated with x-ray lithography. Another attractive feature of x-ray lithography is a near absence of backscattering from the substrate. However, the difficulty and cost of making the x-ray mask, and the problem of distortion in the pattern on the mask due to the stresses in the x-ray absorbing material that forms the pattern, and the lack of stiffness in the membrane that supports the absorber pattern, are considered potential impediments to utilization.
Another potential problem with x-ray lithography, which arises especially when sub-100 nm features are to be replicated, is that the mask-substrate gap, G, must be decreased in accordance with the approximate relationship: EQU G=.alpha.W.sup.2 /.lambda. (1)
where W is the minimum feature size, .lambda. is the x-ray wavelength, and .alpha. is in the range 1 to 1.5. For feature sizes below 50 nm, for example, the gap must be below 4 micrometers. Although such gaps, and even mask-substrate contact, are feasible in research, it is questionable whether this would be acceptable in manufacturing. Clearly, it would be desirable to develop a form of x-ray lithography that avoided the necessity of making a mask and the necessity of utilizing a small gap. This is accomplished by the present invention.